This invention relates to optical fibers and, more particularly, to a method for optically monitoring the concentricity of plastic coatings as they are applied to optical fibers.
Plastic coatings that are applied to optical fibers serve multiple purposes. They serve to reduce microbending loss, maintain the pristine strength of the fibers and provide for abrasion and mechanical protection of the fibers during cable manufacturing processes. In addition, the plastic coated fiber may decrease cross-talk between optical fibers and improve the long term stability of optical fibers in an uncontrolled environment. The plastic coating can also serve as the cladding for fused silica cores since the refractive index of many polymer materials is less than that of fused silica.
For optimum performance, the coating must be applied concentrically around the fiber. This is necessary for routine handling and splicing of the fibers as well as for optimum strength and transmission characteristics.
The coatings which include such materials as silicon, epoxy-acrylates, tetrafluoroethylene, perfluoro-vinyl-methyl ether, perfluoronated-ethylene-propylene, and ethylene-vinyl-acetate copolymer, are applied to fibers by various methods. Prior art techniques for examining the concentricity of coatings once they are produced require the microscopic examination of the fiber after a run has been processed which is time-consuming and destructive. More importantly, real time information to enable the fabricator to make corrections, evaluate various applicators or stop the process completely is not available as the coating is being applied.
In the copending patent application of Mr. H. M. Presby, Ser. No. 685,527, filed May 12, 1976, and assigned to the present assignee, and which issued on Aug. 16, 1977 as U.S. Pat. No. 4,042,723, the concentricity of a plastic coating as it is applied to an optical fiber is determined by comparing the locations of intensity peaks in the backscattered light patterns generated in response to two orthogonal light beams. The present invention is an alternate and improved method for determining concentricity.